You have to buy them. I forget how much, but most houses buy it by the bushel where it is cheaper and includes a barcode as part of the replication package.
You can call around to get one - try replication houses, mastering, etc. They'll probably make you a deal, since they buy them by the 50 or 100s.
As said above, it is a way to keep track of your product. Most record stores and on-line shops require it since it makes their accounting easier. And it is the mark of the devil, too, but don't let that bother you none.
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